CREATE TABLE IF NOT EXISTS  stu(

    id INT(11) PRIMARY KEY AUTO_INCREMENT,//主键 自增

    stuname VARCHAR(10) NOT NULL,//非空

    sex CHAR(2),

    age INT(10) DEFAULT 23,//默认

    seat INT(10) UNIQUE,//独一无二

    classid INT,

    CONSTRAINT fk_stu_classid FOREIGN KEY(classid) REFERENCES class(id) //外键

);